The present invention relates generally to vehicle powertrains, and more particularly to a power take-off unit for a transaxle equipped with a unique worm/worm gear transmission.
Many four-wheel drive vehicles are equipped with a transfer unit which distributes motive power from the powertrain to the front and rear axles in order to drive all of the wheels. For example, in front-wheel drive vehicles, a power take-off unit (PTU) is used to transfer power from the transaxle to the rear driveline. One known type of PTU includes a transfer shaft driven by the transaxle and an intermediate shaft which is driven by the transfer shaft. Typically, the intermediate shaft is provided with a bevel gear which meshingly engages a second bevel gear mounted on the PTU output shaft which drives the rear driveline. A problem with current bevel-type transfer units is that the bevel gears are large and can generate undesirable noise.
Accordingly, it is desirable in the art of vehicle powertrains to provide an alternative to current transfer units which are smaller and quieter than current designs.
Worm gearsets, otherwise referred to as worm/worm gear transmissions, are known in the mechanical power transmission field. The worm gear is driven by the rotation of the worm with which it meshes. The rotational speed of the associated shaft of the worm gear is a function of the number of teeth on the worm gear and the number of threads on the worm. The worm may be single or multiple threaded. Conventional worm/worm gear transmissions have a worm gear with at least 24 teeth. In particular, the American National Standard xe2x80x9cDesign of Industrial Double-Enveloping Wormgearsxe2x80x9d (ANSI/AGMA-6030-C87) recommends 24 as the minimum number of gear teeth. Furthermore, the enveloping angle of known worm gears for one revolution of the thread of the worm is not more than 15 degrees. Moreover, standard double enveloping worm/worm gear transmissions have been used only for ratios of five and more. Due to such high ratios, it has been considered impractical to use the worm gear as the driven member and the worm as the driving member to transfer power from the worm gear to the worm.
It is an object of the present invention to provide a power take-off unit (PTU) for a transaxle which is quieter and more compact than prior designs. These and other objects of the present invention are obtained by providing a power take-off unit with a worm gearset including a transfer shaft rotatably driven by a carrier of a front differential assembly. The transfer shaft drives a transfer gear which is meshed with an intermediate gear mounted on an intermediate shaft. The intermediate shaft also includes one of a worm or worm gear which matingly engages the other of a worm and worm gearset which is mounted to a power take-off unit output shaft. The worm gearset is a double enveloping worm/worm gear transmission which has a very low ratio between the number of gear teeth on the worm gear and the number of threads on the worm. Preferably, the number of worm gear teeth is less than twenty-four.
Enveloping worm/worm gear transmissions with a worm gear having less than twenty-four teeth have not been commercially used because it was believed impossible to build such a transmission due to the need to undercut the root of the worm gear tooth. Thus, those skilled in the art did not consider enveloping type worm gears with less than twenty-four teeth to be feasible for commercial applications. In contrast, the enveloping worm/worm gear transmissions of the present invention utilize a worm gear without undercut gear teeth because of a greater enveloping angle for one revolution of the worm thread.
With less than twenty-four gear teeth and a greater enveloping angle for one revolution of the thread, as compared to prior enveloping worm/worm gear transmissions, the minimum ratio for one thread could be reduced to two, with an increased efficiency. In contrast, prior enveloping worm/worm gear transmissions had a minimum ratio of twenty-four for one thread of the worm and a ratio of five for five threads of the worm. The efficiency of the new worm/worm gear transmission is even greater than that of well-known hypoid gearsets which are used in low ratio right angle drives. Thus, the present invention can replace hypoid or bevel gearing in many applications by reason of the low ratio. In addition, this new worm/worm gear transmission is able to back drive by transmitting torque from the worm gear to the worm. For the same size, this invention has more than twice the capacity of traditional hypoid gearing.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.